This quest can only be done during ‘Beyond Hill and Dale’, the story book land you visit with Syanna. There is a notice board next to the Little Flint Girl that you can look at. This contract is on it.
Go to the Goose’s home as marked on the map. Use your witcher sense to find clumps of feathers on the ground. Follow them until you encounter a group of pixies. Fight them and then continue following the trail to a bandit camp.
Fight the 3 dwarf bandits and loot them for the key (it’s on Red Beard). Free the goose and it will lay you a golden egg before waddling off. If you eat the egg you’ll gain an ability point.